The present invention relates generally to the field of circuitry for protecting “target devices” from electrostatic events (for example, protection against ESD (electrostatic discharge) and more particularly to ESD protection for magnetoresistive (MR) sensors.
ESD protection circuitry protects a “target device” from electrostatic events, such as electrostatic discharge. One known type of target device is magnetoresistive (MR) sensors, such as anisotropic (AMR), giant magnetoresistive (GMR), and tunneling MR (TMR) sensors. MR sensors are typically used in the magnetic storage devices to read data written on magnetic media, and are known to be highly sensitive to ESD damage. For high density storage devices such as hard disk drives (HDD), GMR and TMR sensors are typically destroyed by ESD levels of less than 1 V (volt) and down to 0.5 V and below. One known device to protect sensitive MR devices from ESD damage is called a “dissipative shunt.” The dissipative shunt is useful at reducing charges on the target MR device to avoid ESD damage, but the dissipative shunt does not directly protect the device against ESD pulses.
There are several different types of diodes used in various kinds of conventional ESD protection circuitry. These diode types include: Schottky diodes, PN diodes and diode-wired (also called diode-connected) field-effect transistors (see definition of “diode types,” below). Diode-wired field-effect transistors (DWFETs) are typically made by: (i) connecting the drain and source of a JFET (junction gate FET) and using the gate and source-drain as the two diode terminals; or (ii) connecting the gate and drain of a MOSFET (metal oxide semiconductor FET) and using the source-gate and drain as the two diode terminals. Herein, “DWFET” shall be construed to mean any diode that uses a FET in any manner (now known or to be developed in the future).
In conventional ESD protection circuitry, a diode is connected in parallel with the target device in order to shunt current past the target device when the voltage levels are above the normal operating point of the target device. In another known ESD protection structure (i) a first set of diode(s) in series with the target device (herein referred to a “series-connected diode set); and (ii) a second set of diode(s) in parallel with the element formed by the series diodes with the target device (herein referred to as a “parallel-connected diode set”). It is known that a series-connected diode and a parallel-connected diode can have different “characteristics,” such as carrier concentrations, dopant level, dopant types and band gap levels. However, conventional ESD protection circuitry uses only a single “diode type” for all of the diode(s) in both the series-connected diode set and the parallel-connected diode set.